The dehalogenation of polybrominated or polychlorinated acetic acids, the selective electrochemical substitution of halogen in haloacrylic acids by hydrogen or deuterium, the preparation of fluorinated acrylic acids by electrochemical dehalogenation of chlorine-containing precursors or the electrochemical preparation of unsaturated halogenated hydrocarbons involves cathodic dehalogenation reactions in the presence of catalytic amounts of metallic salts such as, for example, lead acetate, copper nitrate and others. The reactions are carried out in partitioned electrolysis cells in the presence of water at current densities of up to 8000 A/m.sup.2 under strongly acidic conditions. At the anode, aqueous or alcoholic hydrohalic acids are oxidized to elemental halogen. Hazardous substances having in some cases appreciable toxicity, such as halogenated acetic acids, fluorinated acrylic acid or fluorinated halogenated hydrocarbons can therefore be electrolyzed. The hydrogen bromide or hydrogen chloride eliminated in the electrolysis processes or the halogen formed anodically form aggressive electrolytes which act extremely corrosively towards metals. Examples of such electrolytes are mixtures of mono- and dichloroacetic acid and concentrated hydrochloric acid or chlorine and concentrated hydrochloric acid. The electrolytes have very good electrical conductivity. The processes are carried out at temperatures of up to 90.degree. C. and in some cases under pressures of up to 10 bar. Graphite is preferably used as electrode material.
To carry out such dehalogenation processes on an industrial scale, partitioned flow-type electrolysis cells are preferably considered. Owing to their sealed construction, for example, flow-type electrolysis cells make it possible to electrolyze toxic, aggressive and otherwise hazardous substances. Furthermore, high electrolyte flow rates in the region of about 0.5-2 m/sec relative to the electrodes can be achieved in flow-type electrolysis cells. The most common design of a partitioned flow-type electrolysis cell is the plate-and-frame cell. It is composed essentially of usually rectangular electrode plates and frames made of plastic, for example polyethylene, polypropylene, polyvinyl chloride or polyvinylidene fluoride, which surround them. The electrode plate and the associated frame are frequently joined to each other to form an assembly unit. By pressing a plurality of such plate-and-frame units together, a stack which is assembled according to the constructional fashion of filter presses is obtained. Yet further frame units, for example for receiving spacing gauzes or turbulence generators, can be inserted in the stack. The membranes used to partition the cells into anode or cathode compartments are either inserted in a separate frame or clamped directly between the electrode frames. In the case of electrolysis in flow-type electrolysis cells constructed in filter press fashion, in which electrolytes having high electrical conductivity are used, electrical energy is usually supplied by a monopolar parallel connection of the electrodes, i.e. the electrical current is fed to every individual cathode or anode via a separate current supply leads. For this purpose, the anode plates and cathode plates are fed through between the frames on opposite sides of the cell in each case and are provided with current connections outside the cell.
Such plate-and-frame cells have the disadvantage, however, that the forces occurring on pressing the cell stack together have to be absorbed by the frames made of plastic materials which are not particularly dimensionally stable. Under continuous loading, in particular at elevated working temperatures and elevated working pressures, with higher pressure forces being necessary in turn to compensate for the latter, deformation (creep) of these materials can easily occur. In addition, the electrolytes encountered in organic electrosyntheses bring about, depending on the nature and concentration of the organic compounds contained therein, swelling or material embrittlement in the case of many of the conventional plastics, which results in distortion of the cell frames. In addition, appreciable changes in length, which can be compensated for only with difficulty by the conventional disk springs or helical springs of the pressure device occur as a consequence of the relatively high coefficients of thermal expansion (approximately a factor of 10 compared with graphite) of the plastics conventionally used for manufacturing the cell frames, in particular in the longitudinal direction of the cell stack. The above-mentioned properties of the plastics therefore frequently result in leaks in the cell stack, with high safety, health and environmental risks; they make expensive measures such as catching troughs and gas extraction devices necessary. A further disadvantage in the case of the known plate-and-frame cells is the more difficult reassembly and resealing of the cell stack after repair or after the routine replacement of electrodes or membranes if the cell frames have lost their dimensional accuracy as a result of the deformation, swelling or distortion described above.